Monitoring BEMF is a key factor in controlling BLDC motors. The zero cross point of the BEMF provides an indication of the rotor position. FIG. 1 diagrammatically illustrates a typical conventional example of a BLDC motor apparatus. The U, V and W phase inputs of the BLDC motor 13 are driven by an arrangement of drive transistors 12. A sinusoidal controller 11 provides control signals 14-19 for the drive transistor arrangement. A BEMF zero cross determiner 10 determines the BEMF zero cross point based on available feedback information. When a BEMF zero cross occurs, the determiner 10 provides a zero cross indication to an input 9 of the controller 11.
One conventional approach to BEMF determination is direct detection of BEMF using a window-opening method. In most BLDC motors, only two phases carry drive current at any time, and the third phase is floating. This opens a window to detect the BEMF in the floating winding. Window-opening has disadvantages such as relatively low efficiency and control torque distortion, and is typically ineffective in applications where noise is an important concern.
Another conventional approach is indirect estimation of BEMF using a windowless method. The windowless method makes calculations that are highly dependent on the accuracy with which the motor current is sensed. This dependency on accurate motor current sensing narrows the range of applications for the windowless method. For example, during low speed and low current operation, the motor current may be too low for accurate sensing. With core-less motors, which have very low inductance, high amplitude current ripple makes accurate current sensing even more difficult.
It is desirable in view of the foregoing to provide for BEMF zero cross determination in BLDC motors, while avoiding disadvantages associated with conventional approaches.